The present invention relates to a DVD reading-writing pickup head employing multiple wavelengths, more particularly to a DVD reading-writing pickup head employing a laser source which generates a laser beam with multiple wavelengths.
It is known that optical pickup heads are used to record and reproduce information such as video or audio data on and from optical data recording media. As the storage capacity of optical disks is increased, the configuration of pickup heads is also needed to be modified; for example, the wavelength of the laser beam used to read/write data onto optical disks is varied. In the format of the 650 MB CDs, the laser sources with 780 nm wavelength are used. In the format of the 4.7 GB DVD-ROMs, the laser wavelength of the laser sources is 650 nm. In addition, the first generation DVD-R uses the 650 nm-wavelength laser source for reading and the 630 nm-wavelength laser source for writing, and the next generation DVD will use the 410 nm-wavelength laser source.
For the compatibility of the pickup heads for CD and DVD etc., it is necessary that a new pickup head is capable of generating a laser beam with multiple wavelengths for the different disk formats such as the wavelengths 780 nm, 650 nm, 630 nm and 410 nm. FIG. 1 shows a pickup head employing multiple wavelengths according to the related art. A plurality of chromatic beam splitters 90a, 90b and 90c are used to reflect the laser beams with different wavelengths and split the reflected laser beams with different wavelengths onto photo detectors 91a, 91b, 91c and 91d. The configuration of the pickup head of the related art needs a lot of optical elements, and it induces more difficulty of assembly.
The major objective of the invention is to provide a pickup head employing multiple wavelengths with least optical elements.
Another objective of the present invention is to provide a pickup head employing multiple wavelengths with only one photo detector for transferring optical signals to electric signals.
According to the present invention, a cemented birefringent prism and a plurality of quarter-wave plates for different wavelengths are placed in the light path of the pickup head employing multiple wavelengths. The cemented birefringent prism mixes a plurality of laser beams with different wavelengths and same polarization generated by a plurality of laser sources to an incident laser beam, and splits the incident laser beam and a reflected laser beam from an optical medium such as a CD. With the configuration of the pickup head according to the present invention, the chromatic beam splitters of the related art are not required and only one photo detector is needed.
In the present invention, the cemented birefringent prism consists of two birefringent prisms with optical axes perpendicular to each other, which each birefringent prism is made of double-refraction materials. The cemented birefringent prism of the present invention is one of the Rochon prism and the Sernamont prism. Double-refraction material induces different refraction indices corresponding to different polarizations of incident laser beams. Consequently, a plurality of incident laser beams with same polarization traveling through a cemented birefringent prism with definite incidence angles can be refracted and mixed into an optical branch. In addition, a cemented birefringent prism is used to split laser beams which return from an optical medium to the same detector. The chromatic beam splitters and photo detectors of the related art are replaced with a cemented birefringent prism and a photo detector in the pickup head of the present invention.